THE CHLOROPHYLL APPARATUS 157 
any formation or liberation of hydrogen ever been detected so 
long as the plant is maintained in normal conditions. 
The formation of formaldehyde, again, is very difficult 
of proof. It very readily undergoes change, and therefore 
is difficult to detect in a plant. It has been found, how- 
ever, that if Spirogyra is fed with a compound of form- 
aldehyde and sodium-hydrogen-sulphite, which slowly 
evolves the former in the presence of water, a formation 
of carbohydrates occurs. This cannot, however, be accepted 
as proof that formaldehyde normally subserves this purpose. 
There is, however, a certain amount of evidence that 
formaldehyde plays some part in photosynthesis. Bouilhac 
and Tréboux have succeeded in getting plants to grow in 
a very dilute solution of it. Moreover, formaldehyde has 
been obtained from plants by distilling leaves which have 
been exposed for a long time to light and subsequently 
soaked in water. Even in these experiments, however, it 
is not certain how it was produced. In any except very 
dilute solutions it is intensely poisonous to plants. 
Within the last few years formaldehyde has been detected 
in leaves which have been plucked after exposure for some 
hours to a bright sun. The test was devised by Mulliken, 
Brown, and French, and is extremely delicate. To apply it 
take 1 ¢.c. of a 5 per cent. solution of gallic acid in absolute 
alcohol, add about 8 ¢.c. of pure concentrated sulphuric 
acid so that the two fluids do not mix, and allow a small 
quantity of the suspected extract of the leaves to stream 
down the side of the test tube in which the experiment is 
being conducted. The presence of formaldehyde will be 
indicated by the formation of a blue-green ring at the zone 
of contact of the upper and lower liquids. 
If we concede that formaldehyde is very probably the 
first stage in the photosynthetic process, a consideration of 
the probable decomposition seems to lead us to the view 
that the carbon dioxide and the water are made to interact 
without the liberation of carbon monoxide, and that the 
reaction may be represented by the equation CO; + H,0 = 
